The long-term goal of my research is to elucidate the molecular mechanisms underlying genetic instability of malignant cells. The high frequency of recombinations in cancer cells causes genetic instability and generates a large pool of variant tumor cells. Because of this large pool, there is increased probability for selection of drug resistant or malignant clones that have growth advantage and/or aggressive behavior. In spite of its potentially important role in pathogenesis and therapy of cancer, very little is known about the machinery (enzymes, accessory proteins, nucleotide sequences) or mechanisms causing genetic instability. The best example of genetic instability is provided by human acute lymphocytic leukemias where V(D)J recombinase-mediated inappropriate recombinations cause high frequency of translocations, deletions, and inversions. Therefore, we will use the V(D)J recombinase and a DNA translocation substrate as a model system to examine the molecular basis of genetic instability. The V(D)J recombinase mediated translocations fuse protooncogenes into T-cell receptor/immunoglobulin genes, increase or alter their expression, and thus, foster leukemogenesis in humans. We plan (i) to identify and characterize the nucleotide sequences and proteins involved in these translocations and (ii) to determine the role of agents used in cancer therapy, such as radiation and DNA alkylating agents, in causing genetic instability through increased recombinations/translocations in our model system. We will also produce transgenic mice expressing high levels of V(D)J recombinase. These mice are expected to directly demonstrate the role of V(D)J recombinase in genetic instability/leukemogenesis. Additionally, translocations caused by V(D)J recombinase may allow us to identify new protooncogenes. Genetic recombination is a key mechanism for creating genetic diversity. However, inappropriate recombinations can cause altered or increased expression of protooncogenes and/or genetic instability. During our investigations, we expect to identify and characterize nucleotide sequences and proteins that may facilitate inappropriate recombinations causing genetic instability.